Satellite communications systems, employing at least one or more satellites, are widely used to provide both fixed wireless and mobile wireless communication services to a geographically dispersed population of end-user terminals/handsets.
A satellite communications system may utilize a single satellite antenna beam (herein referred to as antenna patterns, spot beams, or more simply beams or cells) covering the entire area served by the satellite communication system. Alternatively, satellite communications systems may employ multiple satellite beams, each of which provides service to distinct geographical areas contained within an overall service region, to collectively serve a larger overall satellite footprint. The satellite typically communicates with terminals or handsets over a bidirectional communications link, with communications signals being communicated from the satellite to a terminal/handset over a downlink or forward service link, and from the terminal/handset to the satellite over an uplink or return service link. The overall design and operation of satellite communication systems and methods are well known to those having skill in the art, and need not be described further herein.
Satellite communication systems and methods may deploy tens or hundreds of antenna beams or service cells, each of which corresponds to one or more spot beams, over a satellite footprint corresponding to the satellite service area. Large numbers of beams/cells may be generally desirable, since the frequency reuse and the resulting system capacity of a satellite communications system may increase in direct proportion to the total number of beams/cells employed. Moreover, for a given satellite footprint or service area, increasing the number of antenna beams may also provide a higher antenna gain per cell, which in turn can increase the link robustness and improve the quality of service.
The ensemble of forward beams/cells may be spatially congruent, substantially congruent, or substantially incongruent with the set of return-link beams. The entire satellite footprint, or substantially the entire satellite footprint, or a part of the satellite footprint may be covered by the set of forward and/or return-link beams. The locations and contours of the forward and/or return-link beams may be a priori determined and fixed, or may be reconfigurable relative to a given geographic location in response to a measurement and/or set of measurements. In support of these measurements the satellite forward and/or return links may be configured to transmit and/or receive at least one calibration signal upon which one or specific calibration measurements may be performed. In the case of the forward-link beams, the measurements may be made by equipment located on the satellite, or by calibration terminals/equipment located on the ground at known geographic locations. In the case of the return links, the necessary calibration signals may be generated by terminals and/or equipment located on the ground, and the measurements may be performed by either the satellite or satellite gateway. These forward and return link measurements are generally referred to as the beamforming calibration process, and are used to initialize and/or update and refine the performance of the satellite forward and/or return link beams, and are considered to be an integral part of the overall beamforming process.
The generation of a large number of forward link service beams is frequently enabled by the use of a phased array antenna onboard the satellite. Such phased array antennas typically utilize multiple, simultaneously radiating antenna elements driven by signals that are individually weighted in both phase and amplitude in order to properly point and shape the resulting beam set. The design of the phased array antenna may be either direct radiating or indirect-radiating. In a typical direct radiating phased array antenna, the radiating elements directly illuminate the service area or earth. In typical indirect phased array, an intermediate reflecting aperture is used to focus and/or shape the patterns of the radiating elements.
In order to provide adequate signal levels on forward link service beams, each of the individual input signals applied to the phased array elements on the satellite typically may be suitably pre-amplified prior to insertion onto the phased array antenna elements. In order to equalize the signal input levels applied to the individual transmit amplifiers, and therefore maintain the amplifiers within their proper signal level range, Hybrid Matrix Amplifier (HMA) configurations are commonly used onboard communication satellites.
A typical Hybrid Matrix Amplifier is comprised of a set of N (N=2n, where n is an integer) parallel amplifiers located symmetrically between two, cascaded N-input by N-output multi-port hybrid matrix (MPHM) devices. In a typical HMA arrangement, N individual amplifier input signals are supplied by the N outputs of the N×N Input MPHM, and the N amplifier output signals are similarly applied to the input section of the N×N Output MPHM.
Typical Multi-Port Hybrid Matrix devices use specific arrangements of interconnected hybrid couplers to combine N input signals to produce N output signals, where the resultant outputs signals are linear combinations of the N input signals. The amplitude and phase (or time delay) weightings that are internally applied by the MPHM to the input signals in order to create the N output signals are typically functions of the MPHM's specific design. Owing to the nature of the signal transformation performed by the HMA, the intermediate N signals located at the input (and output) of the parallel amplifiers are usually substantially identical in amplitude. This equalization of amplitude/drive levels across a group of intermediate power amplifiers is one reason for incorporating HMAs on board communication satellites. In a typical satellite beamforming application, both the Input MPHM and the Output MPHM are co-located on the satellite.
The above description has focused on communications between the communication satellite and the wireless terminals/handsets. However, fixed and mobile satellite communications systems also commonly generally employ a bidirectional feeder link (or links) for communications between a satellite gateway, or gateways (located on the earth) and the satellite. A typical bidirectional feeder link includes a forward feeder link from the gateway to the satellite and a return feeder link from the satellite to the gateway. The forward feeder link and the return feeder link may each use one or more feeder link carriers and/or channels located within the feeder links assigned band of frequencies.
FIG. 1 provides a system level overview of a satellite communication system that incorporates traditional satellite-based beamforming, and depicts interconnectivity. The components include a satellite 10, one or more satellite earth station(s) or gateways 20 (including feeder link antennas), and end-user terminals or handsets 30. In this system design, the signals for each individual beam 15 are commonly multiplexed directly onto the feeder link uplink. The satellite 10 may perform feeder link de-multiplexing of signals received at one or more feeder link antennas 11, and beamformer processing to properly amplitude and phase weight the individual phased-array element signals prior to applying the resulting amplitude/phased weighted signals to the phased array antenna elements 12 for the forward and return link beams 15.
FIG. 2 provides a high-level system level overview of a conventional satellite for a system that uses ground-based beamforming (GBBF), and depicts the interconnectivity between the functional beamforming components located within the satellite earth station or gateway. System components may include one or more calibration stations 40, a satellite 10, and one or more satellite earth stations (gateways) 20. These gateways 20 may include one or more feeder link antennas 22, and the Ground Based Beamforming (GBBF) signal processing hardware 24. In this GBBF implementation, the individual signals that are to be radiated by the phased array antenna elements may be pre-weighted on the ground in amplitude and/or phase, and antenna element signals may be multiplexed directly onto the feeder link uplink. In this approach, the individual signals that are multiplexed onto the feeder link uplink may contain constituent or component signals from many, or possibly all, of the final user beams.
FIG. 3 depicts typical conventional GBBF gateway processing for a satellite system that employs GBBF. In this scenario, the user data associated with each beam typically is externally supplied to a GBBF sub-system 50 by additional subsystems located within the satellite gateway. The GBFF sub-system 50 includes a signal replication and forward beamforming weighting unit 52, and element combiner 54, a calibration signal generator 54 and a forward GBBF (F-GBBF) controller 58. Assuming that there are NB beams and NE phased array elements, as many as NB×NE individual signals may be subjected to amplitude and phase adjustment within the GBBF subsystem 50. The GBBF subsystem 50 processes injected calibration signals to derive element specific amplitude and phase corrections, which are applied individually to each of the NB×NE component element signals. At the output of the element combiner 56, each set of NB elemental signals associated with a particular phased array antenna element are summed and the resulting NE composite signals are then multiplexed onto the forward feeder link by a forward link multiplexer 60.
FIG. 4 depicts one possible implementation of satellite payload processing for a conventional system that employs GBBF. In this implementation, NE individual element signals are multiplexed onto the forward feeder link by the satellite gateway, and a satellite payload 70 in turn performs the necessary signal de-multiplexing. The resulting element signals are then individually amplified by the satellite payload 70. In this implementation, each element signal is individually amplified by a single dedicated amplifier. The resulting amplified signals are then applied to the appropriate phased array element from the set of NE elements of an antenna feed array 80.
FIG. 5 depicts another possible implementation of a satellite payload 70′ for a system that employs GBBF. In this conventional implementation, groupings of N individual amplifiers are now replaced by an N-input×N-output HMA 72. In this implementation, elements of the HMA 72 (Input MPHM, N amplifiers, and the Output MPHM) are co-located on-board the satellite.
FIG. 6 depicts a representative 4×4 MPHM 600 showing a typical interconnection of individual hybrid couplers 610. It should be noted that other possible internal processing arrangements are possible. It should be further noted that lower or higher order implementations of MPHMs are possible. For example, 8-input×8-output MPHM arrangements may be employed.
FIG. 7 depicts an alternate 4×4 MPHM 600′ commonly referred to as a Butler Matrix. The Butler Matrix 600′ is similar in design to the MPHM 600 depicted in FIG. 6, with the exception that two 45° phase shifters 620 have been inserted on two of the internal signal paths between the left hand side (input) and right hand side (output) hybrid couplers 610.
FIG. 8 depicts an example of a full 4×4 HMA 800, along with internal interconnections. The HMA 800 includes an input MPHM 810 and an output MPHM 820 are depicted, along with 4 parallel amplifiers 830. This is a more detailed representation of the full HMA that is depicted in the satellite payload of FIG. 5.